Not applicable.
Vitamin E is an essential component of mammalian diets. Epidemiological evidence indicates that Vitamin E supplementation results in decreased risk for cardiovascular disease and cancer, aids in immune function, and generally prevents or slows a number of degenerative disease processes in humans (Traber and Sies, Annu. Rev. Nutr. 16:321-347, 1996). Vitamin E functions in stabilizing the lipid bilayer of biological membranes (Skrypin and Kagan, Biochim. Biophys. Acta 815:209 1995; Kagan, N.Y. Acad. Sci. p 121, 1989; Gomez-Fernandez et al., Ann. N.Y. Acad. Sci. p 109, 1989), reducing polyunsaturated fatty acid (PUFA) free radicals generated by lipid oxidation (Fukuzawa et al., Lipids 17: 511-513, 1982), and quenching singlet oxygen species (Fryer, Plant Cell Environ. 15(4):381-392, 1992).
Vitamin E, or xcex1-tocopherol, belongs to a class of lipid-soluble antioxidants that includes xcex1, xcex2, xcex3, and xcex4-tocopherols and xcex1, xcex2, xcex3, and xcex4-tocotrienols, and related compounds known as plastoquinones. Although xcex1, xcex2, xcex3, and xcex4-tocopherols and xcex1, xcex2, xcex3, and xcex4-tocotrienols are sometimes referred to collectively as xe2x80x9cVitamin Exe2x80x9d in the popular press, Vitamin E is properly defined chemically solely as xcex1-tocopherol. Of the various tocopherols present in foodstuff, xcex1-tocopherol is the most significant for human health both because it is the most bioactive of the tocopherols and also because it is the tocopherol most readily absorbed and retained by the body (Traber and Sies, Annu. Rev. Nutr. 16:321-347, 1996). The in vivo antioxidant activity of xcex1-tocopherol is higher than the antioxidant activities of xcex2, xcex3, and xcex4-tocopherol (Kamal-Eldin and Appelqzvist Lipids 31:671-701, 1996).
Only plants and certain other photosynthetic organisms, including cyanobacteria, synthesize tocopherols. Therefore, dietary tocopherols are obtained almost exclusively from plants. Plant tissues vary considerably in total tocopherol content and tocopherol composition. The predominant tocopherol in green, photosynthetic plant tissues is xcex1-tocopherol. Leaf tissue can contain from 10-50 xcexcg total tocopherols/gram fresh weight.
Non-green plant tissues and organs exhibit a wider range of both total tocopherol levels and tocopherol compositions. In general, most of the major food staple crops (e.g., rice, corn, wheat, potato) produce low to extremely low levels of total tocopherols, of which only a small percentage is xcex1-tocopherol (Hess, Vitamin E, xcex1-tocopherol, In Antioxidants in Higher Plants, R. Alscher and J. Hess, Eds. 1993, CRC Press, Boca Raton. pp 111-134). Oil seed crops generally contain much higher levels of total tocopherols; however, xcex1-tocopherol is present only as a minor component and xcex2, xcex3, and xcex4-tocopherols predominate (Taylor and Barnes, Chemy Ind., Oct.:722-726, 1981).
Daily dietary intake of 15-30 mg of vitamin E is recommended to obtain optimal plasma xcex1-tocopherol levels. It is quite difficult to achieve this level of vitamin E intake from the average American diet. For example, one could obtain the recommended daily allowance of Vitamin E by daily consumption of over 750 grams of spinach leaves (in which xcex1-tocopherol comprises 60% of total tocopherols) or 200-400 grams of soybean oil.
One alternative to relying on diet alone to obtain the recommended levels of vitamin E is to take a vitamin E supplement. However, most vitamin E supplements are synthetic vitamin E having six stereoisomers, whereas natural vitamin E vitamin is a single isomer. Furthermore, supplements tend to be relatively expensive, and the general population is disinclined to take vitamin supplements on a regular basis.
Although tocopherol function in plants has been less extensively studied than tocopherol function in mammalian systems, it is likely that the analogous functions performed by tocopherols in animals also occur in plants. In general, plant tocopherol levels have been found to increase with increases in various stresses, especially oxidative stress. Increased xcex1-tocopherol levels in crops are associated with enhanced stability and extended shelf life of fresh and processed plant products (Peterson, Cereal-Chem 72(1):21-24, 1995; Ball, Fat-soluble vitamin assays in food analysis. A comprehensive review. London: Elsevier Science Publishers LTD, 1988).
Vitamin E supplementation of swine, beef, and poultry feeds has been shown to significantly increase meat quality and extend the shelf life of post-processed meat products by retarding post-processing lipid oxidation, which contributes to the formation of undesirable flavor components (Ball, supra 1988; Sante and Lacourt, J. Sci. Food Agric. 65(4):503-507, 1994; Buckley et al., J. of Animal Science 73:3122-3130, 1995).
The related compounds plastoquinone (sometimes referred to here as PQ) and the tocopherols are the most abundant quinones produced in the plastids of plant cells. These compounds are synthesized in a common pathway, which is illustrated in FIG. 1. PQ is an important component of the chloroplast photosynthetic electron transport chain and accounts for as much as 50% of the total quinone levels in the plastids of plant cells. As can be appreciated from the synthetic pathway, the levels of PQ and tocopherols are related by the enzymes which catalyze the production of the precursors of both classes of compounds.
The present invention is based on an isolated DNA fragment including a coding sequence for a 2-methyl-6-phytylplastoquinol/2 methyl-6-solanylplastoquinol-9 methyltransferase in a transgenic plant.
The invention is also a heterologous genetic construct comprising a 2-methyl-6-phytylplastoquinol/2-methyl-6-solanylplastoquinol-9 methyltransferase coding sequence operably connected to a plant, bacterial, or fungal promoter not natively associated with the coding sequence.
Another aspect of the present invention is a method of altering the tocopherol profile of a plant comprising the steps of: (a) providing a heterologous genetic construct comprising a 2-methyl-6-phytylplastoquinol/2-methyl-6-solanylplastoquinol-9 methyltransferase coding sequence operably connected to a plant promoter not natively associated with the coding sequence; and (b) introducing the construct into the genome of a plant.
The present invention is also directed toward transgenic plants which have an altered ratio of xcex4-tocopherol to xcex3-tocopherol and xcex1-tocopherol to xcex2-tocopherol, thus increasing the nutritive value of the plants and products therefrom for human and animals.
In another embodiment, the invention is a plant comprising in its genome a heterologous genetic construct comprising a 2-methyl-6-phytylplastoquinol/2-methyl-6-solanylplastoquinol-9 methyltransferase coding sequence operably connected to a promoter that is functional in plants.
It is an object of this invention to provide a plant having an altered xcex4-tocopherol to xcex3-tocopherol ratio.
Other objects, features, and advantages of the invention will become apparent upon review of the specification and claims.